Conventionally, Japanese Patent Application Publication No. 2006-202494 (JP2006-202494 hereinafter) (see FIG. 21, Paragraphs [0108] and [0111]) discloses an LED color-adjusting/dimming lighting device capable of varying a color temperature and brightness of white light by combining two types of LEDs having complementary colors, e.g., blue-green LEDs and brown LEDs, and operating a first lighting circuit allowing a predetermined current to flow in an LED series circuit of a first luminous color and a second lighting circuit allowing a predetermined current to flow in an LED series circuit of a second luminous color with independent duty cycle control.
Japanese Patent Application Publication No. 2002-203988 (JP2002-203988 hereinafter) (see FIGS. 1 to 4) discloses a lighting device of semiconductor light emitting elements, capable of controlling an average current flowing in an LED series circuit by intermittently performing an oscillation operation of a switching power supply circuit for driving the LED series circuit.
Japanese Patent Application Publication No. 2000-173304 (see FIGS. 11 and 12, Paragraph [0086]) discloses a configuration in which a logical-product of a high frequency square wave signal and a low frequency square wave signal is carried out by an AND circuit and an output signal of the AND circuit is supplied to a control electrode of a switching element which controls a current flowing in an LED series circuit. Such configuration relates to an LED marker lamp which has a dimming function and a blinking function, but does not have a color-adjusting function.
In JP2006-202494, the first lighting circuit allowing a predetermined current to flow in the LED series circuit of the first luminous color and the second lighting circuit allowing a predetermined current to flow in the LED series circuit of the second luminous color are configured as constant current circuits using bipolar transistors, whose circuit loss for a constant current control is high and efficiency is low.
In case of using the switching power supply circuit as disclosed in JP2002-203988, it may be possible to improve the efficiency. However, controlling two types of LED series circuits is not taken into consideration. The switching power supply circuit in JP2002-203988 includes a high frequency oscillation circuit which controls a switching element to be turned on at a predetermined frequency, a peak current control circuit which controls the switching element to be turned off when the current flowing in the switching element reaches a predetermined value, and an oscillation control circuit which intermittently stops the oscillation of the high frequency oscillation circuit when the average current flowing in the LED series circuit is higher than a dimming target value. As such, the configuration is complicated and since the oscillation of the high frequency oscillation circuit is intermittently stopped, one high frequency oscillation circuit cannot be commonly used to control two types of LED series circuits.
Accordingly, in case of directly applying the technology in JP2002-203988 to control of two types of LED series circuits having different luminous colors as disclosed in, e.g., JP2002-203988, the configuration is complicated, which causes an increase in cost.